Reloader: Add support for watching and decompressing Prometheus configuration directories

[TheSpiritXIII]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

The reloader now supports sub-configuration directories. For example, you may want to split your configuration into separate scrape configurations:

scrape_config_files:
- configs/foo.yaml
- configs/bar.yaml

Imagine input/foo.yaml and input/bar.yaml are compressed. We could add to our Reloader:

reloader.New(logger, reg, &reloader.Options{
    CfgFile: "config.yaml",
    SubCfgs: []reloader.SubConfigOptions{
        {
            Dir: "input", // input/foo.yaml and input/bar.yaml are compressed.
            OutputDir: "configs", // `foo.yaml` and `input.yaml` are decompressed here.
        }
    }
})

Verification

Unit tests:

go test ./pkg/reloader/

[bwplotka]

Good job, I think this is solid, just I would maybe change the naming of the sub cfg field.

Thanks! Nice tests 💪🏽

[bwplotka]

Separate hashes for all? 🤔 given normalization can be bit expensive PLUS have side effects (env vars)

[TheSpiritXIII]

Do you mean you want a separate hash for each config directory? Since this method is fairly idempotent (in terms of handling state instead of individual events, a la controller-runtime), I don't think it makes much of a difference. However, I have made this change. Let me know if this is not what you meant. Thanks!

[saswatamcode]

Thanks for the great tests! Generally LGTM, just a couple of minor comments!